What follows is a passage through nearly 350 million
years in the history of the planet earth. The various geological periods
mentioned below
are not just strange names; they represent the divisions of geological time through
which the evolution of the dinosaurs and their ancestors can be traced through the rock record... Click
below to select the period that interests you most, or just scroll down through
the geological time periods...

The ancestry of all land animals, including the dinosaurs,
can be traced back over 400 million years to the Devonian Period.
During this period some animals moved to land from the water. The
reason for this is that the land had food resources that had not
been exploited.
A Devonian lobe finned fish called Eusthenopteron evolved
the limb bones in its fins that were later necessary for the transition
to land. Early
amphibians,
such as Ichthyostega and Acanthostega were
probably closely related to the lobe finned fish Panderichthys,
(perhaps the ancestor of all land-living animals). The evolutionary tree below
shows the possible relationships of these early fish.

THE CARBONIFEROUS PERIOD (about 360 to 290 million years ago)

THE TETRAPODS RADIATE

The main phase of evolution for the
tetrapods
occurred during the
Carboniferous. A wide range of early tetrapods are known,
such as the aquatic Diadectes
and the bizzare Diplocaulus with its boomerang shaped head. Large
terrestrial amphibians such as Eryops also evolved. Reptiles evolved from amphibians when some forms such as Seymouria
'split' from the amphibian lineage. This 'splitting' pattern explains how amphibians and reptiles were
able to evolve seperately along different 'branches', and is the
reason why reptiles and amphibians exist together today, even though
one evolved from the other. Seymouria possessed characters that were
intermediate between amphibians and reptiles. A major evolutionary step occurred
with the development of the
amniotes,
animals able to lay shelled eggs, as this
allowed animals to reproduce out of water for the first time. The evolutionary tree
below shows the relationships of the early tetrapods.

During the
Permian, reptiles populated the land. At this time,
most of the continents were at high, rather cold latitudes. The
'mammal-like' reptiles (pelycosaurs) included such well known
forms as the 'sail backs' Dimetrodon and Edaphosaurus (up to 3m
long) and were the most common tetrapod in the Early Permian fauna.
Other groups living at this time include the
therapsids, which
were a diverse group including Moschops (up to 5m long) and Dicynodon.
It is important to realise that the lineage of animals that
eventually led to the dinosaurs (known as the
diapsids)
were rare
in the Carboniferous and Permian - they showed no sign of their
later rise to dominance. They included forms such as Protorosaurus,
and the remarkable Coeleurosauravus that was able to glide from
tree to tree using skin covered ribs in a similar way to the modern
'flying lizard' (Draco).

At the end of the Permian, the greatest
mass extinction (so far)
in the history of life wiped out 75% of all tetrapod families. The
cause of the mass extinction is not known for certain, but the lineage
of animals that led to the dinosaurs, along with some other groups,
survived. This great extinction marked the end of the
Palaeozoic,
one of the great
geological divisions of time. The Mesozoic era that followed it was to see
the evolution of the dinosaurs themselves. The Mesozoic is divided into three 'periods'
of geological time when the dinosaurs lived. These periods are the
Triassic, Jurassic and Cretaceous.

Those archosaurs most closely related to the
dinosaurs are forms such as Marasuchus. The detailed evolutionary
relationships are still debated, but by the late Triassic, several
early theropods are known, as the dinosaurs rapidly diversified.
These dinosaurs, such as Eoraptor, Coelophysis and Herrerasaurus
were all carnivores, and, despite their diversity, were quite rare
at this time.

The rise to prominence of the dinosaur lineage, it is suggested,
took place as the early dinosaurs took advantage of habitats left
vacant after other extinctions at the end of the Triassic period.
Environmental changes at this time affected other reptiles more
than dinosaurs. This dominance of the land by the dinosaurs was to
last throughout the remainder of the Mesozoic. To understand the further
development of the dinosaurs, it is necessary to look at the
classification of the dinosaurs.

During the Triassic, the carnivorous saurischian dinosaurs such
as the sleek Coelophysis (meaning 'hollow form', reflecting the bone
structure) and the larger and more robust Herrerasaurus occupied the
niche previously occupied by the carnivorous archosaurs.
Other saurischians showed an evolutionary trend towards greater size,
such as the prosauropodPlateosaurus, and became adapted to a herbivorous diet.

During the Jurassic period, the supercontinent Pangaea was beginning
to break up due to the mechanisms of plate tectonics (or 'continental
drift'). This allowed narrow seaways to spread between the continents, but
land links still existed that allowed the dinosaurs to spread throughout
the continent. The extinction of the mammal-like reptiles
at the Triassic-Jurassic boundary allowed the dinosaurs to diversify
rapidly in the Early Jurassic.

The mild Jurassic climate (though still warm) allowed the largest
dinosaurs to evolve. The prosauropods continued into the Jurassic and
showed an increase in size (a common trend in the fossil record). The 10m long sauropodVulcanodon shows
numerous changes to the skeletal structure (for example, in the hip)
to accomodate the great increase in weight. This animal laid the foundation
for the true dinosaur giants, which evolved early in the Jurassic and became
distributed worldwide. From these animals, more advanced sauropods such as
Brachiosaurus evolved, which had a very long neck and elongated forelimbs,
an adaptation to reach high vegetation. Other sauropods
were adapted to browsing on lower vegetation. The largest of all sauropods,
(such as Seismosaurus) are also of Jurassic age.

Though the sauropods were saurischian, other herbivores at this time
evolved from the ornithischian line, such as Stegosaurus and Hypsilophodon.
Large ornithischian herbivores such as Scelidosaurus evolved a turtle-like
'beak' for the close cropping of low plants. Primitive iguanodonts such as
Camptosaurus also showed features adapted to low grazing.

The largest carnivore of the early Jurassic was the double-crested
Dilophosaurus, (which DID NOT have a 'frill' or spit poison as a well
known film would have us believe!) up to 6m in length, which evolved from
earlier ceratosaurs such as Coelophysis. The carnosaurs were much
larger predators, and include animals such as Allosaurus.

THE CRETACEOUS PERIOD (about 150 to 65 million years ago)

THE GREATEST DIVERSITY OF DINOSAURS

By the Cretaceous, the distribution of the continents was becoming more
similar to the present day. The climate was very warm, the Atlantic ocean
opened, and the seaways were more extensive. The separation of landmasses
isolated certain groups of dinosaurs and allowed them to evolve separately
from each other as they were unable to breed and exchange genes. This isolation allowed
the tremendous diversity of Cretaceous dinosaurs to evolve.

The large saurischian sauropods of the Jurassic were rare, however.
The majority of the herbivores that replaced them were smaller
ornithischian dinosaurs which showed a greater diversity of form,
the ornithopods. The replacement of the sauropods by the ornithopods
probably reflects the influence of environmental stresses on the large
sauropods. Perhaps the large sauropods
may have had some difficulty in adapting to a diet of flowering
plants that evolved in the Cretaceous.
Iguanodon was
the largest of the ornithopods, and in the early
Cretaceous evolved a wide
variety of specialisations in related to its diet. Large herbivores
such as Tenontosaurus were closely related. The nodosaurs
were the successors to the Jurassic stegosaurs, and, like the iguanodonts
developed a horny 'beak' for grazing on low plants.

Cretaceous carnivores included the ostrich-like Ornithomimus, and the 3m
long theropod Deinonychus. The teeth and bones of a Deinonychus were found near
the bones of a much larger Tenontosaurus, which may suggest they preyed on
the ornithopod. Deinonychus evolved several specialised structures associated
with its carnivorous way of life, including a large retractable
12cm claw on the second toe, used to slash at prey animals.

During the Late Cretaceous, the largest terrestrial carnivores of all
evolved, such as Tyrannosaurus (up to 14m long), the tiny, but very
strong
forelimbs of which were specialised; their purpose remains debated.
Equally large, though less well known was Carcharodontosaurus.
The great diversity of form that evolved among Cretaceous theropods
is underlined by Carnotaurus, which evolved a short, bulldog-like snout,
and two horns over the eyes.

Also at this time, the 'duck-billed' dinosaurs (the hadrosaurs and
lambeosaurs) came to prominence. These were related to the iguanodonts and filled
a similar niche, as low level grazers. Lambeosaurs became very diverse and
the skull developed many different
'ornamental' crests. These elaborate crests may have been used as visual
signalling devices (rather similar to the way modern peacocks use their tails). One group evolved
hollow crests, and this has led some authors to suggest that they were used
as a kind of 'trombone' to add sound to the visual display, for example,
Parasaurolophus. The well known horned dinosaurs, such as Triceratops evolved from the
beaked, Early Cretaceous Psittacosaurus. The elaborate horn development
may have served as a defensive weapon, a signalling device, or may have
been used during rutting contests, similar to the antlers of modern deer.